Growing Pains
by SoldLotus
Summary: Frisk had lived on her own for nine years after releasing all the monsters from the Underworld, but now that she's grown and, for the most part, mature, she's decided to move back in with her Momster, Asriel, and Asgore, but things turns spicy when Sans finally reaches monster puberty.
1. Chapter 1

18 Year Old Female Frisk

Sexual Content Soon to Come

Post-Pacifist End Where Frisk Returned Home Instead of Staying With the Monsters

A/N: So I based Sans birthday when William Caslon IV had created his own sans-serif alphabet. Read bottom notes for more details.

/\/\/\/\/\

Frisk hummed as she looked onto a piece of paper with neat scribblings of a house address and the words "Toriel" above it.

It's been nine years since she had successfully released the monsters and they had now been living semi-peacefully with the humans as a few conflicts were noted here and there about racism from each, but it mainly worked out.

She smiled happily whistling as she rolled her suitcase close behind her. She had returned home after all the ruckus, to only find a note left behind. She frowned at the thought, but shook her head as determination filled her with hope.

Frisk lived nine years on her own as her parents seemed to still pay the bills while school was free and she'd salvage some of the free lunch for dinner and breakfast the next day, while every weekend she'd volunteer at the homeless shelter giving herself another free meal.

Frisk always thought of going back to her monster family, but her determination and pride always got in the way as she already told Toriel that she'd had places to be.

She stopped walking when she saw a familiar sign. "Grillby's..?"

Without even hesitating, she entered the bar to see the same regulars she's ever seen from the Underworld.

The first two to realize her entrance were both Greater and Lesser Dog, who quickly made their way to her while disarming themselves, but then they stopped a meter from her in uncertainty.

Frisk quickly set herself and her stuff in a nearby booth as she beckoned them both over, and they both seemed excited again.

This has caught the eyes of the rest of the customers and the grill master himself, as everyone else knew better than to sit anywhere the dogs could reach you.

"They're being pet.." Dogamy.

"(I wanna be pet, too..)" Dogaressa.

"Pet? Where?" Doggo.

"That's cute.." Drunken rabbit thing.

"They look like they're having fun." Venus fly trap thing.

She giggled before letting the two dogs down though they whined. "I can't pet you anymore. Remember what happened to your neck last time?"

Frisk got her stuff as the dogs reluctantly returned to their places, though the rest just started at her as she gave them all a pat on the head before actually heading to the bar.

She smiled at the man with the head of fire before changing to sign language. "[Could I have a burger, Grillby?]"

His fire seemed to hiss in appreciation and recognition. "[If it isn't the human from the Underworld. We'll continue when I bring your food.]"

She nodded happily when she was suddenly surrounded by the occupants of the bar. "You're the weird puppy, aren't ya?" Dogamy asked being the first one to speak as he was sniffing her.

Frisk laughed slightly nervous as she nodded her head. "(I thought I recognized your smell.)" Dogaressa stated slightly happy.

"You petted us." Doggo said in glee as he felt his head.

"Hehe, yeah. It's been a while, has it not?"

That was when Grillby came out with her burger and a bottle of ketchup along with some sort of drink.

"[It's on the house.]" Grillby signed before leaning on the bar counter. "[So where have you been all this time]"

Frisk answered out loud so everyone could understand her response in case they were wondering. "Well after we were freed from the Underworld, I thought it'd be best to return to my home before it all happened and finish school before coming back, so here I am." She chuckled motioning to herself. "I just hope Toriel and the rest aren't to surprised by my reappearance."

The human downed some of whatever beverage Grillby has given her and started eating the burger slowly.

"Actually, Sans is usually here around this time.." the bird monster who use to translate for Grillby thought out loud.

"Aw, is someone getting bone-ly without me?"

Frisk jumped when the seat beside her was suddenly filled with a short man wearing a blue jacket and- wait a minute.

"..Sans?" The human asked perplexed as she stared at him. He looked exactly the same.

The said skeleton turned to face her with his ever so chipper smile, despite what seemed to be a raised skeleton equivalent of an eyebrow to show slight confusion.

"Do I know you?" He looked her over and his smile grew a bit. "Tibia honest, it's pretty guts-y for you to come to a monster town."

Frisk laughed softly at the puns and the fact that he didn't change a bit before tackling him out of happiness. "Haha, it really is you! You haven't changed one bit!"

The married dogs chuckled as they all returned to their usual seats accompanied by Grillby, who's fire seemed to flicker with amusement.

"W-wha- Lady, I uh think you got the wrong sack of bones." He replied still shocked motionless as the light disappeared from his eye sockets and sweat formed on the back of his skull.

She hugged him tightly before getting off of him with one final giggle of happiness. "No, I definitely have the right lazy bones."

Frisk helped up the still shocked, though calmed down, skeleton before handing him her bottle of ketchup and finishing off her burger.

"I guess you wouldn't recognize me being older and whatnot.. it has been nine years since we last saw each other." She gave a small disappointed smile as she finished off her drink as well.

.-.-.-.-.-.-.-.-.-.-.

Sans accepted the ketchup and looked over to the woman beside him.

He had to admit that she did look somewhat familiar, though he couldn't quite lay a finger on it.

She had said that they had last seen each other nine years ago..

Nine years ago?

Wasn't that around the time they first made it to the surface?

But the only human he knew around that time- oh.

"Say, what did you say your name was, kid?"

She perked at the use of her nickname used only by Sans, who braced himself since she looked like she was going to tackle him again.

The human gave a huge smile (which was absolutely astonishing in San's mind) and responded with evident happiness in her voice. "Frisk. My name is Frisk."

Sans awkwardly scratched the back of his skull as he remember what she looked like as a kid and tried to avoid looking her over once again. "Well, you've certainly grown in many ways.."

Grillby, who was still nearby, quickly turned his flame towards Sans in a disapproving manner, immediately making the shorter one apologize embarrassingly.

Frisk, who seemed oblivious to it all, laughed softly. "Guess between us, I don't look much like the baby bones anymore."

Grillby seemed to find her joke amusing while Sans recollected himself and put a dramatic hand to his chest. "I beg to differ, I have a good hundred or so years on you."

Frisk looked shocked. She knew monsters aged differently, but Sans? He's over a hundred? Frisk suddenly felt bad for Papyrus.

She directed her attention to Grillby and started a signed conversation with him, confusing the skeleton. "[How old is he?]"

"[199 to be exact. His birthday was on March 6, but I guess each ten years is about a year on his bones. Each monster ages differently.]"

"And he's older than Papyrus?" She asked out loud gaining San's attempted interruption.

"That doesn't really matter, does it-?"

Grillby shook his head indicating that Sans was indeed the younger brother.

Sans sunk in his chair in slight embarrassment at being younger as Frisk laughed.

And goodness, it didn't seem like she was going to quiet down anytime soon.

/\/\/\/\/\

CONTINUING THE ABOVE NOTES

I based San's birthday year off of when William Caslon IV had created a sans-serif alphabet, because Sans had used a sans-serif in his serious speeches (or so I believe). It's just a theory that his comic side to him is nearly a façade as we know he owns a time traveling machine in working progress. So like there's all the stuff about Gaster as he's worked with Sans and Alphys way back when as suggested when talking to Alphys at the end of the true pacifist end, and known about Gaster from the two passing people who spoke of his incident. But anyways, I was debating on doing sans-serif or comic sans because comic sans was recently made in 1994 or 1995 then I thought of using dejavu sans which is used for his snoring (which actually might suggest him being all knowing and whatnot) but I couldn't find a date on it. So yeah sorry if this doesn't make sense cause I'm tired, but the meaning behind all this is that IT ISN'T ACTUALLY AN OFFICIAL BIRTHDAY ye. As for the month and day, they're random.

Please review, follow, favorite, and let me know whatcha think! Thanks!


	2. Chapter 2

Sans and Frisk walked together through the streets as several monsters whispered to each other about the human that the skeleton was walking with.

He couldn't tell if it bothered Frisk or not seeing as she stayed chipper humming to herself, so he decided to ignore them as well.

The skeleton sighed. He stood at an even five foot and was about four inches shorter than the human despite him being much older.

He hated the fact that he was so short and that he knew skeletons only grew for a certain amount of years before they stayed the same, and he hasn't grown for at least 150 of them.

He jumped as he felt squishy flesh grab his hand. "Are you alright, Sans?"

The said monster took a deep breath to calm himself from the blush that threatened to creep up. "I'm fine."

He squeezed her hand back in hopes that she wouldn't realize anything.

She stared at him before bringing her other hand to her mouth. "You're haha you're voice just.."

He smacked his phalanges to cover his teeth and nose bone as his magic didn't bother to make its rushing to his face discreet.

"Ah! Sorry, I didn't mean to embarrass you or anything..."

Sans really hated this part of his life. He doesn't understand why he had to hit puberty so late and why his voice wouldn't stop betraying him and why he had a rush of sudden urges and why he couldn't stop looking Frisk over and-

"Guess you could say I'm still bone tired from staying up all night." Sans replied shifting his eyes away from the human beside him.

She giggled lightly before swinging their arms and continuing to walk. "I completely understand. I couldn't sleep all night thinking of meeting everyone. I'm still nervous.. but I'm happy I have you here right now. It kinda gives me a bit more confidence."

He looked shocked before giving a big smile, just like Frisk remembered them being when she was younger. "Well that's good because we're practically there."

Frisk's smile faltered a little along with her grip on his hand. "Really? That was fast.."

He chuckled and squeezed her hand again to let her know that he was still here for her. "Hey, don't sweat. I'll just go in and let them know that I brought a surprise. Pap will be clavicle when he sees you."

Frisk gave him a small smile that sent magic crawling up his spine as he imagined being given the smile right after having- Sans gulped, swallowing as he tried to get rid of the thought. "Thank you so much, Sans."

He nodded as he refused to trust his mouth to welcome her properly. He faced the door and easily turned the knob.

"What took you so long, lazy bones!" A fish like monster with flowing crimson hair laughed out as she was obviously in the middle of giving a tall skeleton a noogie.

As the words left her mouth, the skeleton that was being assaulted turned to face the other, much shorter, skeleton standing in the doorway. "BROTHER! I, THE GREAT PAPYRUS, REQUIRE YOUR ASSISTANCE!"

"Ok, bro." Was all that the other replied with and did nothing as he saw a yellow dinosaur-like monster calm the fish one.

"Well." Sans cleared his throat after a while of watching the monsters, effectively gaining their attention. "I have a surprise that just might rattle your bones.. but where's Tori and Asgore?"

"T-they're still setting the table for dinner." The yellow dinosaur said and perked with slight interest as she looked intently at Sans.

Papyrus jumped happily and clapped his hands earning a small giggle from the human that was off to the side, though none other than Sans heard it over the taller skeleton's yelling. "WHAT IS IT?! IS IT A PUPPY?"

"Yeah, just show us already!" The fish yelled equally as loud, if not louder, than Papyrus.

"Well, let's not get so frisky." Sans chuckled at his joke and nudged the girl beside him as if to ask if she was ready, and she returned the gesture with a hesitant nod.

He moved to the side and gave a dramatic gesture towards her as she walked forward.

There was a silence in the room for a good ten seconds making Frisk even more nervous as she even tried to turn to leave, if it wasn't for the skeleton standing behind her.

"Congratulations!" The fish woman, Frisk remembered her to be Undyne, yelled and attempted to grab the confused monster behind Frisk to give him a noogie, but he swiftly dodged. "I can't believe lazy bones here actually managed to find a nice catch!"

The dinosaur, Alphys, continued after Undyne had stopped in place to howl in laughter. "When's the m-marriage?! Oh my goodness, I already ship it so hard!"

Papyrus' magical eyes buldged from his eye sockets as he ran and picked up the girl and twirled her around. "I SEE YOU'VE FOUND A SUITABLE SECOND BEST, HUMAN! WE'LL REALLY BE RELATED NOW!"

He squished her into a hug as she just questioned everything that happened up till now. "BUT I MUST KNOW, WHERE HAVE YOU BEEN ALL THESE YEARS?"

Frisk laughed silently before happily hugging him again. "I've missed your silliness all these years, Papyrus." She replied ignoring his question, though his question had caught the other two's attention.

"You know her, Papyrus?" Undyne said shocked before looking at the embarrassed blue man. "How long has this been going on?"

"What's all the ruckus about-" a goat woman entered wearing a violet tunic-like dress. Papyrus put the human down with a huge grin on his face while watching her place her bag down and looked the queen over in an unexplainable sense of happiness.

The goat woman took off her gloves as she stared back at the now-grown girl as tears filled her eyes. "...Frisk?"

Upon hearing her name, she ran and hugged the woman, who still stood at least a foot taller than the human. "Mom! I missed you." Frisk laughed softly as she cuddled in the soft and warm embrace of her monster mother.

"You didn't tell me you were visiting.. how long will you be here?" Frisk smiled before exiting the embrace to hold her mother's hand and look her in the face. "I.. I actually was planning on moving in across the street."

"Nonsense! You can live here if you so wanted." Toriel insisted looking hopeful at her daughter.

Frisk looked up at her in thought. "Well, I wouldn't mind as long as I could bring a few things.. like two pet ferrets?"

Toriel nodded as she happily hugged her long lost human child, and that was when Asgore decided to show up.

"Tori.. I can't make the pie by myself.." a golden bearded goat monster walked out saying in a slight whine.

"Wait.." He looked at the two before smiling brightly. "Frisk, right? It's great having you back."

Frisk sheepishly nodded at the monster king as she hasn't spoken with him much.

It seems that Undyne and Alphys had finally realized what was going on as the fish yelled out. "You're the punk?!"

"Frisk, a-are you really marrying Sans? I j-just.. I never expected that.. b-but it's still very shipable!" Alphys said as her face seemed to brighten at the thought.

That made all Frisk, Toriel, and Asgore sputter in disbelief while both Frisk and Sans also blushed at the notion.

"I didn't- I.. What?" Frisk said stumbling on her words as she covered her face in embarrassment.

Sans silently took in her flustered face, and quickly stocked on ways he could see it again, when Asgore and Toriel stared intently at the skeleton while the king seemed to send killer vibes his way. This caused the shortest of the bunch (even Alphys stood at least an inch above him) to sweat profusely.

He knew that Asgore had wanted to become family once again with Toriel, and after succeeding that, he'd hoped for another child, then BANG! Here's Frisk again.

"We, uh, we didn't seal any deal. She's still fresh meat." Sans said in a poor attempt at keeping his cool.

Undyne then grabbed Frisk and started giving her a noogie, which caught the teen by surprise. "I can't believe you only came back to tell us that you two are engaged, ya punk!"

"Ah! Stop, please!" Frisk said getting really red faced at the action she hadn't faced in a long time. "Undyne!"

Sans swear he felt his breath hitch as his magic stopped zooming through his bones for a whole second. Frisk definitely matured into a beauty as her caramel locks flowed down each curve of her body, around her most-likely-c-cup breasts, falling right under her curvaceous ass, which was perfectly tensed as she bent ever so slightly with a pleading expression glued to her ever so flustered red face as she begged Undyne to stop in embarrassment and pain.

'I wouldn't mind a little girl action between the two- or even better, with me-' Sans cursed his mind for wondering in a forbidden direction as he felt the hot buzzing of his magic start to gather in a region much lower than his skull.

He hated to admit it, but spending a few years on the Surface had been a time well spent on all the monsters.

The skeleton wasn't talking about Asgore taming his fur or both the skeletons getting smoother, cleaner bones either. He was talking about human cosmetics making every female, and selective other monsters, more mature and defined. He was talking about the human clothes that seemed to show off every sexual appeal any of them had.

He was talking about Undyne's crimson hair cascading at a greater length as it is usually let down instead of in a ponytail, and if that wasn't already making her twice as beautiful as she once was, human cosmetics and clothes did a wonder as the clothes would show off her well defined muscles.

He was talking about Toriel's new use of glasses and amazingly well groomed fur as she always portrayed her teacher attitude and wore stereotypical teacher clothing.

He was talking about Alphys' adorable nerding out as she could be seen wearing many different types of cosplay and was definitely more confident in herself.

He swallowed a breath of air as he mentally kicked himself. 'You're supposed to be avoiding this, bone head!'

"-right, Sans?" He jumped as he looked around to see Papyrus holding up Frisk and she turned to him with hopes he'd help her out.

"yup." Was the skeleton's usual reply as he had no idea what she said, though he figured it had something to do with the earlier conversation.

It was then where everyone calmed down (though Sans would beg to differ as he successfully hid the dim blue glow under his jacket as he sat down on the couch and cursed himself repeatedly) and chatted with Frisk asking many questions.

Toriel then stood back up with Asgore following shortly behind. "I'd love to stay longer and chat, and even help you out with settling into your new room, but I have a teacher conference and King Flulffybuns here has a political meeting to go to."

Frisk gave her a pout before waving with a small smile. "Bye Mom, Asgore.. I'll be here when you return so don't worry." She got up and hugged the woman before speaking again, but more quietly. "Stay safe, okay?"

The queen just patted her head affectionately before handing her over to Asgore, of which him and Frisk only gave an awkward one armed hug which he awkwardly initiated as the two exchanged a small laughter afterwards.

"I hope the skeleton brothers would be overjoyed to help you til we come back."

Sans opened his eye sockets a bit wider as if he was going to make an excuse to leave when Toriel gave him a stern look.

"I, THE GREAT PAPYRUS, AND MY BROTHER, LAZYBONED SANS, WOULD BOTH GLADLY HELP THE HUMAN WITH WHATEVER NEEDS SHE HAS!" Papyrus declared with a look of pride as Sans sagged as he hoped that his mind wouldn't wonder any farther than it already had.

/\/\/\/\/\

Sorry this was so late

school work doubled for some stupid reason and I hadn't had time to work on all the fanfics I've started Dx

Please review, favorite, and or follow! Thanks, hope you enjoyed


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